With the recent development of digital devices such as computers, it has become common to process an enormous amount of operational data or two- or three-dimensional image data. There is a need to provide large-capacity high-speed memories and high-performance microprocessors for quick processing of such enormous information. Further, the processing power required of the digital devices is estimated to increase more and more as the broadband spreads with the development of network systems such as the Internet.
In order to meet this need, various devices such as semiconductor devices are required to achieve higher density and higher integration. The requirements for photolithography processes, which enable fine patterning, are particularly becoming more stringent year by year. For example, photolithography process using ArF excimer laser radiation (wavelength: 193 nm) has been put into use in response to the requirement for patterning techniques with a minimum line width of 0.13 μm or less for production of 1-Gbit or higher-capacity DRAMs. The development of photolithography process using extreme ultraviolet (EUV) radiation has also been pursued for finer patterning.
Although novolac resins and polyvinylphenol resins are conventionally used in resist compositions, these resins show too high light absorption to be used in the above wavelength ranges. The use of acrylic resins (see e.g. Patent Document 1) and cycloolefin resins (see e.g. Patent Document 2) as alternative resist resins is thus being examined.
The change of light sources and the improvement of exposure apparatuses are also being examined in order to achieve the finer device structures. For example, steppers (reduced projection type exposure apparatuses) have been improved significantly by performance improvements to reduced projection lenses and design modifications to optical systems. The performance of the projection lens used in the stepper is generally expressed as NA (numerical aperture). As the physical limitation of the NA of the projection lens is of the order of 0.9 and has already been accomplished, it has been attempted to increase the NA of the projection lens to be 1.0 or higher by filling any medium of higher refractive index than the air into the space between the lens and wafer. Among others, attentions are being given to liquid immersion exposure processing using pure water (hereinafter sometimes just called water) as the medium (see Non-Patent Document 1).
There are pointed out various problems in liquid immersion lithography as the liquid immersion lithography involves contact of the resist film with the medium (such as water). In particular, the occurrence of changes in pattern shape due to the dissolution of an acid generated in the film by exposure or an amine compound added as a quencher into water and the occurrence of pattern collapse due to the film swelling become issues. It has been reported that it is effective against these problems to improve the water repellency of the resist film for separation of the resist film and water. It has also been reported that it is effective to form a water-repellent top coat film on the resist film (see Non-Patent Document 2).
Various fluorine-containing polymers for fluorine-containing resist materials have been developed as the use of fluorine-containing resist compositions is effective for improvements in resist water repellency. The present applicant has disclosed a difluoroacetic acid having a polymerizable double bond-containing group (see Patent Document 3) and a difluoroacetic acid ester having in its ester moiety an acid-labile protecting group such as t-butyl or methoxymethyl group and an ester group (see Patent Document 4). However, each of these patent documents is intended for use of the difluoroacetic acid or difluoroacetic acid ester as a negative or positive resist and does not disclose an ester polymer having no acid-labile protecting group. As non-polymerizable compounds similar to the above polymerizable compounds, carboxylic acid compounds having a fluorine atom in α-position, such as 2-fluoro-phenylacetic acid and esters thereof (see Patent Document 5) and ethyl-2,2-difluoro-3-hydroxy-3-phenylpropionic acid (see Non-Patent Document 3), are known.
On the other hand, the top coat film for separation of the resist film and water is required to have good solubility in developer, resistance to pure water and capabilities of separating the resist film and water and not damaging the resist film. As a top coat composition satisfying these requirements, there has been developed a composition of a fluorine-containing polymer having a repeating unit formed with two or more hexafluoroisopropylhydroxyl groups. This fluorine-containing polymer composition is reported to have particularly good solubility in developer (see Patent Document 6).
In general, it is required that the top coat composition applied to the resist film for protection of the resist film has good solubility in developer, resistance to pure water and capabilities of separating the resist film and water and not damaging the resist film as mentioned above. If the developer solubility of the top coat composition is low, it is unfavorably impossible to obtain a rectangular cross-section resist pattern due to deterioration in photoresist performance by insufficient removal of the top coat film. If the developer solubility of the resist composition is too high, the resist film may be reduced in thickness. It is thus desirable that the top coat composition shows adequate developer solubility.